Better UX for page bleed and margins

Summary:

After more than a week of working with the Pages Tool, I just discovered that the icon of the text entry in the page margins is actually a button that shows a popover of options. — This is actually a nonsense and it makes it difficult for new users to discover this feature.

We need a better design for this feature to be discoverable.

Adam's suggested splitting that icon into an actual button.

Version info

Inkscape 1.4-dev (3db96bfbac, 2023-07-22)

    GLib version:     2.66.8
    GTK version:      3.24.24
    glibmm version:   2.64.2
    gtkmm version:    3.24.2
    libxml2 version:  2.9.10
    libxslt version:  1.1.34
    Cairo version:    1.16.0
    Pango version:    1.46.2
    HarfBuzz version: 2.7.4

    OS version:       Debian GNU/Linux 11 (bullseye)